1. Field of the Invention
The present invention relates to an optical fiber bundle unit for transmitting ultraviolet light such as that from an excimer laser or the like.
2. Description of Related Art
When an optical fiber having a core doped with germanium dioxide is used for transmitting ultraviolet light, loss increase and the transmittance decrease occur due to Rayleigh scattering, and therefore, the optical fiber cannot be used for transmitting ultraviolet light. Therefore, a pure quartz glass core optical fiber having a pure quartz glass core portion and a cladding portion doped with fluorine is usually used as an optical fiber for transmitting ultraviolet light.
However, when the pure quartz glass core optical fiber is used for irradiating and transmitting ultraviolet light having a high energy level, such as a KrF excimer laser or an ArF excimer laser, the quartz glass included in the optical fiber is affected by optical energy (hν) to generate radicals (defects of the molecular structure in the optical fiber) in accordance with the following formula (1):Si—O—Si+hν→Si.+.O—Si  (1)As a result, the transmittance of the optical fiber decreases.
In order to prevent the decrease of the transmittance, the optical fiber is exposed to an atmosphere including hydrogen so that hydrogen enters the optical fiber, as a result of which radicals generated by irradiating ultraviolet light having a high energy level react with hydrogen to yield Si—OH and Si—H species, which results in the energy state of the optical fiber being stable by repairing defects of the molecular structure in the optical fiber (which are caused by radicals).
Even when the optical fiber is exposed in the hydrogen atmosphere, radicals may not be completely eliminated from the optical fiber. Alternatively, when ultraviolet light having a high irradiation energy is irradiated on the optical fiber subjected to the hydrogen exposure, bonds in the net-like molecular structure of the quartz glass may be cut, generating radicals. At this time, when hydrogen is present in the optical fiber after the hydrogen exposure, the generated radicals immediately react with hydrogen in a stable energy state, as a result of which the increase of the transmission loss can be suppressed.
However, since hydrogen tends to gradually diffuse in the aforementioned conventional optical fiber over time, which reduces the concentration of hydrogen remaining in the optical fiber, it becomes impossible to continue to suppress the increase of the transmission loss over time.